The present invention relates to a radiation source assembly. The radiation assembly includes at least two radiation sources displaced laterally with respect to an optical axis of the assembly and further includes at least one optical diffuser spaced from the radiation sources along the optical axis. The invention also relates to a non-dispersive transducer utilizing such a radiation source assembly and having a radiation detector located to receive the radiation emitted by the radiation sources. The invention finds particular utility in the analysis of gases and gaseous mixtures, although its field of use is not limited to such applications.
The analysis of gases based on non-dispersive infrared absorption, such as the identification of the components of anesthetic and breathing gases and the determination of their concentrations in patient environments, is currently carried out by means of transducers requiring considerable space. Basically there are two types of transducers suitable for such analysis--transducers measuring directly in the mainstream of the patient's breathing circuit and transducers measuring in a small sidestream taken from the breathing circuit.
Sidestream transducers are generally located within a patient monitor, to which gas samples are supplied through a small diameter sampling line. The measurement is made at a distance from the patient so that the transport of the gas samples to the transducer can take a few seconds. This causes a time delay in obtaining the measurement results. It also causes a mixing of consecutive samples, which delays the observation of changes in the measurement results; i.e., an increase of the signal rise time. The advantages of sidestream measurement are the minor importance of transducer size and the short distance between the measurement electronic circuits and the transducer. Corrections to the transducer signal can be easily made, and therefor the concentration readings are usually very accurate and reliable although, as noted above, they are obtained with a delay in signal production and increase of rise time. It is due mainly to these disadvantages of sidestream transducers, that mainstream transducers are becoming more common, particularly in applications where speed and response in the measurement results are important.
Mainstream transducers are connected to the mainstream of the patient's breathing circuit by a special adapter. A mainstream transducer of this kind is described e.g. in U.S. Pat. No. 4,914,720. The transducer comprises a sample chamber or connecting tube, which forms a section of the mainstream channel and which has two opposing windows forming part of a sample chamber. The transducer itself is located outside the windows, so that the transducer's infra-red radiation source is directed from the outside toward one of these windows and through the mainstream channel toward the second window, on the outside of which is placed at least one measurement detector with its narrow bandpass filter. Typically two detectors are used, and a second detector makes reference measurements through a narrow bandpass filter operating at another wavelength band, in order to correct e.g. disturbances occurring in the radiation intensity. The electrical signals from the detectors are supplied via electrical lines to a device which calculates the measurement result.
A disadvantage of transducers of this type is the signal's sensitivity to the erroneous absorption caused by water and mucus, which may accumulate on the windows of the sample chamber. By heating the windows of the sample chamber it is possible to avoid the condensation of water, but this practice can not reliably compensate for the shading and spectral effects of mucus. The reason for this is that the reference detector will see the amount of mucus differently than the measurement detector, because the transmission band of the narrow bandpass filter of the reference detector and that of the measurement detector are on different wavelength ranges, so that the radiation which passes through the mucus will pass through these filters in different ways. Further the measurement and reference beams located generally in parallel may actually have a different geometric distribution in the sample chamber and this can cause differences in the measurement and reference signals.
Another mainstream transducer like the mainstream transducer described above is described in the publication HEWLETT-PACKARD JOURNAL September 1981, pages 3-5,; R. J. Solomon --"A Reliable, Accurate CO.sub.2 Analyzer for Medical Use." In the described transducer, the measurement accuracy is increased, and particularly the drift of the measurement value is reduced, by modulating the infrared radiation passing through the sample chamber with a rotating disk, in which the filters comprise closed cells containing exactly known gaseous mixtures. For example, in a transducer for measuring carbon dioxide, one of the cells contains carbon dioxide. In a solution of this kind, the measurement geometry and reference geometry are approximately equal, and, due to the optical gas filter, the measurement and reference wavelength bands are also equal, as described in the publication thereby overcoming some of the problems associated with the transducer shown in the '720 patent.
However, a transducer of the kind described in the Hewlett-Packard publication is very complicated, expensive, and very sensitive to mechanical shock etc. Further there can occur wear of the bearings of the filter disk, which increases the unreliability and the service costs of the transducer.
The use of an optical gas filter is known e.g. from U.S. Pat. No. 3,745,349. The transducer described in that patent contains two infrared radiation sources, one of which radiates through the other, so that the radiation from both the first and second infrared sources have exactly the same optical path. Between the infrared sources, i.e. in front of the first source, there is an optical filter, which is based on the optical absorption of gas, and which provides a very narrow absorption band at the wavelength of the absorption band of the gas to be measured. The infrared sources are used alternately, so that a detector placed on the opposite side of the sample chamber as seen from the infrared radiation sources receives alternately a measurement signal from the second source and a reference signal from the first source. The beams of radiation from both sources propagate in the same path and see the water and mucus in the sample chamber approximately in the same way because they are of the approximately same wavelength. This structure eliminates a major part of the disadvantages discussed above.
However, the structure of the transducer in U.S. Pat. No. 3,745,349 has such disadvantages that the proposed structure has not found practical use. If conventional long-lasting miniature incandescent lamps with burning time of thousands or tens of thousand hours are used as infrared radiation sources in the transducer, then, due to their slowness, only a very slow alteration of the radiation sources, i.e. a slow modulation, can be realized. The measurement speed achievable with the transducer will not be sufficient. On the other hand, if the filaments are made thinner and smaller to an extent that a sufficiently rapid modulation is possible, the filaments will burn out very quickly, so that the transducer has very low practicality. Errors are also caused by the fact that the filaments have a rather large area in the radiation propagation direction, so that the second gas measurement radiation source shades the first reference radiation source and causes different paths for the beams at the sample chamber and its windows. Moreover, the transducer is so bulky that it can be used only as a sidestream transducer. Therefore the transducer presented in the patent incorporates a fixed sample chamber, and thus is not suited for use on a mainstream transducer, for instance.
An embodiment is also presented in which the second source is not required to transmit the energy received from the first source. This is advantageous from production standpoint and the signal from the first source will also be higher. An optical combining network comprising two intersecting tubes through which the radiation is directed guides the radiation from each source into the sample chamber. However, the described arrangement would have to be very long for the radiation from both sources not to propagate differently through the sample chamber and its windows. This also means that a small reliable transducer cannot be produced in this manner to fulfill the requirements of a mainstream transducer.